igb: Drop BUILD_BUG_ON check from igb_build_rx_buffer

On s390 the igb driver was throwing a build error due to the fact that a frame
built using build_skb would be larger than 2K.  Since this is not likely to
change at any point in the future we are better off just dropping the check
since we already had a check in igb_set_rx_buffer_len that will just disable
the usage of build_skb anyway.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>